JP7785504B2 - Powder and its manufacturing method - Google Patents

Description

This disclosure relates to a powder and a method for producing the same. More specifically, this disclosure relates to a powder containing particles that contain carbon, nitrogen, boron, and oxygen as constituent elements and have the same crystal structure as turbostratic boron nitride, and a method for producing the same.

Boron nitride powder possesses lubricity, high thermal conductivity, and insulating properties, and is widely used as a solid lubricant, thermally conductive filler, insulating filler, and other applications. In recent years, the increasing performance of electronic devices has created a demand for boron nitride with excellent thermal conductivity.

For example, Patent Document 1 proposes a hexagonal boron nitride powder and a method for producing the same, which, when used as a filler for insulating heat dissipation materials such as resins, can increase the thermal conductivity and withstand voltage (dielectric breakdown voltage) of the resins.

Hexagonal boron nitride powder is produced, for example, by a manufacturing method that includes a pressure nitridation process in which boron carbide is nitrided in a pressurized nitrogen-containing atmosphere to obtain a nitride, and a decarburization and crystallization process in which the nitride is mixed with a boron source and heat-treated to decarburize and promote crystallization, thereby obtaining hexagonal boron nitride (see, for example, Patent Document 2). The above-mentioned manufacturing method aims to improve decarburization performance in subsequent processes by nitriding boron carbide once.

Japanese Patent Application Laid-Open No. 2019-116401 International Publication No. 2019/073690

However, at present, the yield of hexagonal boron nitride produced using the above-mentioned production method is not particularly high.

The present disclosure aims to provide a powder containing particles that contain carbon, nitrogen, boron, and oxygen as constituent elements and have the same crystal structure as turbostratic boron nitride, which can improve the productivity of hexagonal boron nitride, and a method for producing the powder.

In order to solve the above-mentioned problems, the present inventors have conducted extensive research and found that, while conventional manufacturing methods employ a method in which boron carbide nitride obtained in a pressure nitriding step is decarburized and crystallized in a subsequent step, the yield of hexagonal boron nitride is not increased in terms of crystallization in this step, and there is room for improvement. The inventors then discovered that by heat-treating boron carbide nitride in an environment where it is likely to come into contact with oxygen, thereby decarburizing it in advance to reduce the amount of residual carbon and adjusting the crystal structure of the nitride to a state of low crystallinity, it is possible to shorten the firing time in the subsequent crystallization step, thereby improving the productivity of hexagonal boron nitride. Furthermore, it has ^been found that, with regard to the crystal structure of the nitride described above, a powder in which the ratio of the maximum peak in the wavenumber range of 1480 cm ⁻¹ or more and 1900 cm ⁻¹ or less to the maximum peak in the wavenumber range of 1000 cm ⁻¹ or more and less than 1480 cm −1 in the Raman spectroscopy spectrum is used as an index, corresponds well to a raw material powder suitable for improving the productivity of hexagonal boron nitride. The present disclosure is based on this finding.

One aspect of the present disclosure provides a powder containing particles having the same crystal structure as turbostratic boron nitride, which contains carbon, nitrogen, boron, and oxygen as constituent elements, and in which, when A is the maximum value of the peak in the wavenumber range of 1480 cm ⁻¹ or more and 1900 cm ⁻¹ or less, and B is the maximum value of the peak in the wavenumber range of 1000 cm ⁻¹ or more and less than 1480 cm ⁻¹ , the value A/B is 1.4 or less in a Raman spectrum.

A powder containing particles having the same crystal structure as turbostratic boron nitride, which contain carbon, nitrogen, boron, and oxygen as constituent elements, has a low A/B value. In the Raman spectroscopy spectrum of particles having the same crystal structure as turbostratic boron nitride, peaks in the wavenumber range of 1480 cm ⁻¹ to 1900 cm ⁻¹ correspond mainly to in-plane vibrations of carbon atoms characteristic of the graphene structure, and are peaks observed when the crystallinity is excellent. On the other hand, peaks corresponding to structural disturbances, defects, etc. in graphene, and peaks corresponding to hexagonal boron nitride, are observed in the wavenumber range of 1000 cm ⁻¹ to less than 1480 cm ⁻¹ . In other words, the low A/B value means that the particles constituting the powder have a sufficiently reduced amount of highly crystalline portions (e.g., residual graphite contained in the raw material boron carbide, etc.), and contain a relatively large amount of amorphous content. This makes the powder suitable as a raw material for manufacturing hexagonal boron nitride to improve productivity.

The powder may have a carbon content of 4.0% by mass or less and an oxygen content of 5.0% by mass or less. By ensuring that the carbon and oxygen contents of the powder are within the above ranges, when the powder is used as a manufacturing raw material, the amount of auxiliary agents used in the subsequent decarburization and crystallization processes can be reduced in advance, and the heat treatment time can be shortened, thereby further improving the productivity of hexagonal boron nitride.

The powder may have an average particle size of 10 to 100 μm. The particle size of the hexagonal boron nitride can be adjusted by adjusting the particle size of the powder. When the powder has an average particle size within this range, the performance of the resulting hexagonal boron nitride as a filler can be improved.

One aspect of the present disclosure provides a method for producing a powder containing particles having the same crystal structure as turbostratic boron nitride, the powder containing carbon, nitrogen, boron, and oxygen as constituent elements. The method comprises a pressure nitriding step in which boron carbide powder is sintered in a pressurized nitrogen atmosphere at a temperature of 1900 to 2200°C to obtain a sintered product, and an oxidation step in which the sintered product is heat-treated in an atmosphere having an oxygen partial pressure of 20% or more to obtain a heat-treated product. The oxidation step includes stirring a mixture containing the sintered product and the heat-treated product.

The method for producing the powder described above includes an oxidation step in which the sintered product obtained in the pressure nitriding step is heat-treated in an oxygen-containing atmosphere, and the mixture is stirred during the oxidation step. By stirring the mixture during the oxidation step, the surface area of the sintered product or heat-treated product that comes into contact with oxygen can be changed, sufficiently reducing the carbon content and other components contained in the sintered product or heat-treated product, while also reducing highly crystalline components in the powder (e.g., residual graphite contained in the boron carbide raw material), and further suppressing the improvement of crystallinity in the particles that make up the resulting powder. A production method that includes such operations can reduce the A/B value of the resulting powder.

The heat treatment temperature in the oxidation step may be 700 to 1050°C.

In the above manufacturing method, the mixture may be stirred using a rotary kiln. When the stirring is performed using a rotary kiln, the powder can be produced more easily than when stirring is performed after temporarily stopping heating, as in batch processing.

The present disclosure provides a powder containing particles that contain carbon, nitrogen, boron, and oxygen as constituent elements and have the same crystal structure as turbostratic boron nitride, which can improve the productivity of hexagonal boron nitride, as well as a method for producing the powder.

Embodiments of the present disclosure are described below. However, the following embodiments are merely examples for explaining the present disclosure and are not intended to limit the present disclosure to the following content.

Unless otherwise specified, the materials exemplified in this specification can be used alone or in combination of two or more. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition refers to the total amount of those multiple substances present in the composition, unless otherwise specified.

One embodiment of the powder according to the present disclosure is a powder containing particles that contain carbon, nitrogen, boron, and oxygen as constituent elements and have the same crystal structure as turbostratic boron nitride. The powder may be an aggregate of the specific particles described above, and may also contain other particles as long as it does not detract from the spirit of the present disclosure. The particles may have the same crystal structure as turbostratic boron nitride (t-BN), and this crystal structure may include, for example, a crystal structure in which some of the elements that make up turbostratic boron nitride are replaced by carbon, or a crystal structure in which carbon is dissolved in a crystal structure made of boron nitride. The particles may also contain particles of what is called boron carbonitride, or may be particles of what is called boron carbonitride.

In the above particles, the total amount of carbon, nitrogen, boron, and oxygen may be, for example, 95% by mass or more, based on the total amount of elements constituting the particles. The composition of the above elements in the above particles may be, for example, a carbon content of 0.1 to 5.0% by mass, a nitrogen content of 48 to 56% by mass, a boron content of 37 to 43% by mass, and an oxygen content of 0.1 to 5.5% by mass.

The crystallinity of the particles constituting the powder is adjusted. In the Raman spectroscopy spectrum of the powder, when A denotes the maximum value of the peak in the wavenumber range of 1480 cm ⁻¹ or more and 1900 cm ⁻¹ or less, and B denotes the maximum value of the peak in the wavenumber range of 1000 cm ⁻¹ or more and less than 1480 cm ⁻¹ , the value of A/B is 1.4 or less. The wavenumber range of 1480 cm ⁻¹ or more and 1900 cm ⁻¹ or less includes peaks corresponding to in-plane vibrations of carbon atoms characteristic of graphene or structures similar thereto, and observed in cases of excellent crystallinity. In the case of graphene, a peak is observed around 1580 cm ⁻¹ . On the other hand, in the wavenumber range of 1000 cm ⁻¹ or more and less than 1480 cm ⁻¹ , peaks corresponding to structural disorders, defects, etc. in graphene, and peaks corresponding to hexagonal boron nitride are observed.

The upper limit of the A/B ratio in the powder may be, for example, 1.3 or less, 1.2 or less, 1.1 or less, or 1.0 or less. When the upper limit of the A/B ratio is within the above range, it is possible to shorten the firing time when the resulting powder is used as a raw material, thereby further improving the productivity of hexagonal boron nitride. The lower limit of the A/B ratio in the powder is not particularly limited, but may be, for example, 0.3 or more, 0.4 or more, or 0.5 or more. When the lower limit of the A/B ratio is within the above range, it is possible to prevent the powder from being excessively oxidized. The A/B ratio in the powder may be adjusted within the above range, and may be, for example, 0.3 to 1.4, 0.4 to 1.3, or 0.5 to 1.3.

The A/B value of the powder is measured by the following method. Specifically, first, a cross-sectional sample of the powder is prepared using a cross-sectional sample preparation device. Next, the obtained cross-sectional sample is used as the measurement target and a Raman spectrum is obtained by measuring it using a Raman spectrometer under the following measurement conditions. In the obtained Raman spectrum, the maximum value A of the peak in the wavenumber range of 1480 cm ⁻¹ to 1900 cm ⁻¹ and the maximum value B of the peak in the wavenumber range of 1000 cm ⁻¹ to less than 1480 cm ⁻¹ are identified, and the A/B value is determined. The measurement is performed by performing mapping measurements in 1 μm steps (10 points x 10 points) on an approximately 10 μm square area at the center of the particle observed in the cross-sectional sample, integrating the spectra for 100 points, and then correcting the baseline derived from fluorescence using a polynomial model to obtain the Raman spectrum. As a cross-sectional sample preparation device, for example, the "CP-9010" (product name) manufactured by JEOL Ltd. can be used. As the Raman spectroscopic device, for example, "Raman Microscope XploRA" (product name) manufactured by Horiba Ltd. can be used.
<Raman spectroscopy measurement conditions>
Laser: 532 nm (neutral density filter: 10%)
Range: 100-4000cm ^-1
Objective lens: 100x Grating: 1200g
Slit: 100 μm
Confocal hole: 100 μm
Laser polarization: Circular
Exposure time: 1 second Number of exposures: 3

The above powder has a reduced carbon and oxygen content. When this powder is used as a raw material for producing hexagonal boron nitride, by reducing the carbon and oxygen to be removed in the powder in advance, it is possible to reduce the amount of auxiliary agents used in the subsequent decarburization and crystallization processes and shorten the heat treatment time, thereby further improving the productivity of hexagonal boron nitride.

The upper limit of the carbon content in the powder may be, for example, 4.0% by mass or less, 3.0% by mass or less, or 2.0% by mass or less, based on the total amount of powder. The lower limit of the carbon content in the powder may be, for example, 0.1% by mass or more, 0.2% by mass or more, 0.3% by mass or more, or 0.5% by mass or more, based on the total amount of powder.

The oxygen content in the powder may be, for example, 5.0% by mass or less, 4.0% by mass or less, 3.0% by mass or less, or 2.0% by mass or less, based on the total amount of powder. The lower limit of the oxygen content in the powder may be, for example, 0.1% by mass or more, 0.2% by mass or more, or 0.3% by mass or more, based on the total amount of powder.

The powder may have a carbon content of 4.0% by mass or less and an oxygen content of 5.0% by mass or less.

The total carbon and oxygen content of the powder may be, for example, 5.0% by mass or less, 3.0% by mass or less, or 2.0% by mass or less, based on the total amount of powder. There is no particular lower limit for the total carbon and oxygen content of the powder, and it may be 0% by mass or less, based on the total amount of powder, but it may be, for example, 0.5% by mass or more, or 1.0% by mass or more.

In this specification, the carbon, nitrogen, boron, and oxygen contents refer to values measured by the following methods. The carbon content refers to a value measured using a carbon/sulfur simultaneous analyzer. An example of a carbon/sulfur simultaneous analyzer that can be used is the IR-412 (trade name) manufactured by LECO. The nitrogen content refers to a value determined by titration. Specifically, the sample is first alkaline decomposed with sodium hydroxide, and ammonia is distilled from the decomposition solution using steam distillation and collected in an aqueous boric acid solution. The nitrogen atom content in the sample can be determined by titrating this collected solution with a normal sulfuric acid solution. The boron content refers to a value determined by titration. Specifically, the sample is first heated and melted in a platinum crucible, completely dissolved in hydrochloric acid, and then titrated with an aqueous sodium hydroxide solution to determine the boron atom content in the sample. The oxygen content refers to a value measured using an oxygen/nitrogen simultaneous analyzer. An example of an oxygen/nitrogen simultaneous analyzer that can be used is the "EMGA-910" (product name) manufactured by Horiba Ltd.

The powder may have an adjusted particle size. The upper limit of the average particle size of the powder may be, for example, 100 μm or less, 90 μm or less, or 80 μm or less. When the upper limit of the average particle size is within the above range, hexagonal boron nitride powder prepared using the powder as a raw material can be suitably used for thin heat dissipation components with a film thickness of approximately 0.2 mm. The lower limit of the average particle size of the powder may be, for example, 10 μm or more, 15 μm or more, or 20 μm or more. When the lower limit of the average particle size is within the above range, the hexagonal boron nitride powder prepared using the powder as a raw material and the heat dissipation components prepared using the hexagonal boron nitride powder can exhibit high heat dissipation properties. The average particle size of the powder may be adjusted within the above range, for example, 10 to 100 μm, 15 to 90 μm, or 20 to 80 μm.

The average particle size referred to herein is a value obtained by measuring the powder as is, without homogenization, and is the average particle size of primary particles, excluding agglomerated particles. The powders described above typically do not contain agglomerated particles. The average particle size referred to herein is also the particle size at which the cumulative value of the volume-based cumulative particle size distribution is 50% (median diameter, d50). The average particle size referred to herein is measured using a laser diffraction/scattering particle size distribution analyzer in accordance with ISO 13320:2009. Specifically, it is measured using the method described in the Examples section of this specification. An example of a laser diffraction/scattering particle size distribution analyzer that can be used is the "LS-13 320" (instrument name) manufactured by Beckman Coulter, Inc.

The above-mentioned powder can be produced, for example, by the following method. One embodiment of the powder production method is a method for producing a powder containing particles having the same crystal structure as turbostratic boron nitride, containing carbon, nitrogen, boron, and oxygen as constituent elements, and comprising a pressure nitriding step in which boron carbide powder is fired at a temperature of 1900 to 2200°C in a pressurized nitrogen atmosphere to obtain a fired product, and an oxidation step in which the fired product is heat-treated in an atmosphere with an oxygen partial pressure of 20% or more to obtain a heat-treated product. The oxidation step is a step that includes stirring a mixture containing the fired product and the heat-treated product (hereinafter also referred to as a stirring oxidation step).

In the pressure nitriding step, the boron carbide powder is fired at a temperature of 1900 to 2200°C in a pressurized nitrogen atmosphere to obtain a fired product. The fired product may contain boron carbonitride (B ₄ CN ₄ ). The lower limit of the firing temperature in the pressure nitriding step may be 1900°C or higher, and may be 2000°C or higher. By setting the lower limit of the firing temperature within the above range, the nitriding of boron carbide can be more sufficiently promoted. Furthermore, the upper limit of the firing temperature may be 2200°C or lower, and may be 2150°C or lower. By setting the upper limit of the firing temperature within the above range, it is possible to prevent the crystallinity of boron carbonitride from becoming too high, and thus to prevent a decrease in production efficiency in the crystallization step, which is a manufacturing process for hexagonal boron nitride powder, when the boron carbonitride powder is used as a raw material for manufacturing hexagonal boron nitride powder. The firing temperature may be adjusted within the above range, for example, 1900 to 2200°C or 1900 to 2150°C.

The lower limit of the pressure in the pressure nitriding process may be, for example, 0.6 MPa or more, 0.7 MPa or more, or 0.8 MPa or more. By setting the lower limit of the pressure within the above range, the nitriding of boron carbide can be more fully promoted. The upper limit of the pressure in the pressure nitriding process may be, for example, 1.0 MPa or less, or 0.9 MPa or less. By setting the upper limit of the pressure within the above range, increases in manufacturing costs can be suppressed. The pressure may be adjusted within the above range, for example, between 0.6 and 1.0 MPa.

The nitrogen gas concentration in the pressurized nitrogen atmosphere in the pressure nitriding process may be, for example, 95.0% by volume or more, 98.0% by volume or more, or 99.9% by volume or more. By keeping the nitrogen gas concentration within this range, boron carbide can be nitrided under milder conditions. The above nitrogen gas concentration is based on the volume under standard conditions. The firing time in the pressure nitriding process is not particularly limited as long as it is within a range that allows sufficient nitriding to proceed, and may be, for example, 6 to 30 hours or 8 to 20 hours.

In the oxidation process, the fired product is heat-treated in an atmosphere with an oxygen partial pressure of 20% or more to obtain a heat-treated product. In the oxidation process, the carbon contained in the fired product is combined with oxygen and removed from the system as carbon dioxide gas, thereby performing a decarburization process. During this process, stirring the mixture containing the fired product and the heat-treated product increases the surface area in contact with oxygen, improving the efficiency of the decarburization process and, by making the decarburization conditions milder, suppressing increases in manufacturing costs.

The atmosphere during the heat treatment in the oxidation step (firing atmosphere) is an atmosphere with an oxygen partial pressure of 20% or more, and may be, for example, air or a mixed gas with an adjusted oxygen partial pressure. From the perspective of reducing production costs, the firing atmosphere is air. The oxidation step may be performed in a closed system or an open system, but adjustments can be made so that the firing atmosphere is maintained. During the oxidation step, air or a mixed gas that meets the above conditions may be circulated at a rate of, for example, 100 L/min or less. During the oxidation step, air or a mixed gas that meets the above conditions may be circulated at a rate of, for example, 20 L/min or more, or 30 L/min or more.

When using a mixed gas, the lower limit of the oxygen partial pressure may be, for example, 20% or more, 25% or more, or 30% or more. A lower limit of the oxygen partial pressure within this range can further promote the carbon oxidation reaction in the oxidation process. When using a mixed gas, the upper limit of the oxygen partial pressure may be, for example, 70% or less, 60% or less, or 50% or less. An upper limit of the oxygen partial pressure within this range can prevent excessive oxidation of the powder. When using a mixed gas, the oxygen partial pressure may be adjusted within the above range, for example, 20-80% or 25-60%. Note that the oxygen partial pressure in this specification refers to the partial pressure of oxygen under standard conditions in the mixed gas occupying the firing atmosphere, as measured by an oxygen concentration meter. An oxygen concentration meter such as the "G1690" (product name) manufactured by Sakaki Corporation can be used.

The upper limit of the temperature (heating temperature) for the heat treatment in the oxidation step may be, for example, 1050°C or lower, 1000°C or lower, or 950°C or lower. Keeping the upper limit of the heating temperature within the above range can prevent excessive oxidation. The lower limit of the temperature for the heat treatment in the oxidation step may be, for example, 700°C or higher, 750°C or higher, or 800°C or higher. Keeping the lower limit of the heating temperature within the above range can further promote the reaction in the carbon oxidation step. The heating temperature in the oxidation step may be adjusted within the above range, for example, 700 to 1050°C, 700 to 1000°C, or 750 to 950°C.

The stirring operation of the mixture containing the fired product and the heat-treated product in the oxidation step may be performed intermittently or continuously throughout the oxidation step. When performed intermittently, for example, the heat treatment may be temporarily stopped for the stirring operation.

The means for stirring the mixture containing the fired product and the heat-treated product in the oxidation step is not particularly limited, and any means capable of changing the surface of the mixture that comes into contact with the firing atmosphere may be used. Examples of such means include manual stirring, stirring using a mechanical stirrer, stirring using a fluidized bed, and stirring using a rotary kiln such as a rotary kiln. The mixture may also be stirred using a rotary kiln. Using a rotary kiln continuously changes the surface of contact with the firing atmosphere while applying vibration to the mixture, which can suppress excessive crystallinity and also facilitate the desorption of carbon dioxide and other gases.

When using a rotary kiln, the raw material, the fired material, is fed into the kiln using a screw feeder or the like, and air or the mixed gas is introduced at a predetermined flow rate, allowing the heat treatment to be carried out in a flowing atmosphere.

The feed rate of the calcined raw material is not particularly limited, but may be, for example, 5 g/min or more, 10 g/min or more, 20 g/min or more, or 100 g/min or more. The feed rate of the calcined raw material may be, for example, less than 1000 g/min or less than 800 g/min. When the upper limit of the feed rate is within the above range, the surface of the calcined raw material can be more fully exposed to oxygen, thereby further improving the production efficiency of hexagonal boron nitride powder using the powder. The feed rate of the calcined raw material may be adjusted by the inner diameter and length of the kiln furnace, and can be set to, for example, approximately 3 to 15 volume % of the volume of the kiln furnace.

The lower limit of the flow rate of air or the mixed gas may be, for example, 10 L/min or more, 20 L/min or more, 30 L/min or more, 40 L/min or more, or 45 L/min or more. By setting the lower limit of the flow rate within the above range, it is possible to ensure a sufficient supply of oxygen while making it easier to remove carbon dioxide and other gases. The upper limit of the flow rate of air or the mixed gas may be, for example, 1000 L/min or less, 950 L/min or less, or 900 L/min or less. Setting the upper limit of the flow rate within the above range can further promote the oxidation reaction of carbon in the oxidation step. The flow rate of air or the mixed gas may be adjusted within the above range, for example, 10 to 1000 L/min, or 20 to 950 L/min.

The lower limit of the rotation speed of the rotary kiln may be, for example, 0.5 rpm or more, 0.7 rpm or more, or 1.0 rpm or more. By setting the lower limit of the rotation speed within the above range, it is possible to adjust the residence time and prevent excessive oxidation and increases in production costs. The upper limit of the rotation speed of the rotary kiln may be, for example, 10 rpm or less, 9 rpm or less, or 8 rpm or less. By setting the upper limit of the rotation speed within the above range, it is possible to further promote the reaction in the carbon oxidation process. The rotation speed of the rotary kiln may be adjusted within the above range, and may be, for example, 0.5 to 10 rpm, or 0.7 to 9 rpm.

The tilt angle of the rotary kiln can be adjusted depending on, for example, the temperature and time of the heat treatment, the feed amount, etc. The tilt angle of the rotary kiln may be, for example, 0.2 to 6°, 0.3 to 5°, or 0.5 to 4°.

The powder obtained as described above is suitable as a raw material for producing hexagonal boron nitride powder. For example, the powder can be mixed with an auxiliary agent such as a boron source to prepare a raw material composition, and the raw material composition can be heat-treated to produce primary particles of boron nitride, resulting in hexagonal boron nitride powder containing agglomerated particles formed by the aggregation of the primary particles. In other words, the powder can be used as a raw material for producing hexagonal boron nitride powder by desorbing and decarbonizing the carbon and oxygen contained in the powder as carbon dioxide gas or the like, generating primary particles of a predetermined size, which are then agglomerated to produce agglomerated particles. One example of a method for producing hexagonal boron nitride powder includes a step (hereinafter also referred to as a crystallization step) of heating a raw material composition containing the powder and a boron source to produce primary particles of boron nitride and obtain agglomerated particles formed by the aggregation of the primary particles.

Examples of boron sources include boric acid, boron oxide, or a mixture thereof. The mixture heated in the crystallization process may also contain known additives.

In the raw material composition, the blending ratio of the powder and the boron source can be appropriately set according to the molar ratio. When using at least one of boric acid and boron oxide as the boron source, for example, the boron source may be blended so that the total amount of boric acid and boron oxide is 20 to 300 parts by mass per 100 parts by mass of the powder, or the boron source may be blended so that the total amount of boric acid and boron oxide is 50 to 250 parts by mass per 100 parts by mass of the powder.

The heating temperature to which the mixture is heated in the crystallization process may be, for example, 2000°C or higher, or 2100°C or higher. By setting the lower limit of the heating temperature within the above range, grain growth can be sufficiently promoted. The heating temperature to which the mixture is heated in the crystallization process may be, for example, 2150°C or lower, or 2100°C or lower. By setting the upper limit of the heating temperature within the above range, yellowing of the hexagonal boron nitride powder can be suppressed. The heating temperature may be adjusted within the above range, and may be, for example, 2000 to 2150°C. The heating temperature to which the raw material composition is heated in the crystallization process is preferably lower than the heating temperature of the powder in the nitriding process.

The crystallization process may be carried out under normal pressure (atmospheric pressure), or may be carried out under pressure exceeding atmospheric pressure (e.g., 50 kPa or higher). When pressure is applied, the pressure may be, for example, 0.5 MPa or less, or 0.3 MPa or less. Note that the above pressure refers to gauge pressure.

The heating time in the crystallization process may be 0.5 hours or more, 1 hour or more, or 3 hours or more. By setting the lower limit of the heating time within the above range, grain growth can be sufficiently promoted. The heating time in the crystallization process may be 40 hours or less, 30 hours or less, 20 hours or less, or 10 hours or less. By setting the upper limit of the heating time within the above range, increases in manufacturing costs can be suppressed. The heating time may be adjusted within the above range, for example, 0.5 to 40 hours, or 1 to 30 hours. Note that in this specification, firing time, heating time, etc. refer to the time (retention time) during which the temperature of the environment surrounding the object is maintained at a specified temperature after it has reached that temperature.

The method for producing hexagonal boron nitride powder may include other processes. Examples of such processes include a pulverization process and a classification process. In the method for producing hexagonal boron nitride powder, for example, a pulverization process may be carried out after the crystallization process. In the pulverization process, a general pulverizer or crusher can be used. For example, a ball mill, a vibration mill, a jet mill, or the like can be used. Note that in this specification, "pulverization" also includes "crushing." The average particle size of the hexagonal boron nitride powder may be adjusted to 15 to 100 μm by pulverization and classification.

Although several embodiments of the present disclosure have been described above, the present disclosure is in no way limited to the above-described embodiments. Furthermore, the descriptions of the above-described embodiments can be mutually applied.

The present disclosure will be described in more detail below using examples and comparative examples. Note that the present disclosure is not limited to the following examples.

Example 1
100 parts by mass of orthoboric acid manufactured by Nippon Denko Co., Ltd. and 35 parts by mass of acetylene black (product name: HS100) manufactured by Denka Co., Ltd. were mixed using a Henschel mixer. The resulting mixture was filled into a graphite crucible and heated in an arc furnace in an argon atmosphere at 2200°C for 5 hours to obtain a fired product containing lumped boron carbide (B ₄ C) (pressure nitriding process). The resulting lumps were coarsely crushed using a jaw crusher to obtain a coarse powder. This coarse powder was further crushed using a ball mill with silicon carbide balls (φ10 mm) to obtain a crushed powder. The crushing using the ball mill was performed at a rotation speed of 20 rpm for 60 minutes. The crushed powder was then classified using a vibrating sieve with a mesh size of 75 μm. The fine powder that fell through the sieve was subjected to airflow classification using a Crusseal classifier to obtain a powdered fired product with a particle size of 10 μm or more. The boron carbide powder thus obtained was subjected to a nitriding treatment in a carbon resistance heating furnace under a nitrogen gas atmosphere at a firing temperature of 2000°C and a pressure of 0.90 MPa for 12 hours.

The nitrided fired powder was fed into a rotary kiln (inner diameter: 100 mm, length: 1900 mm) at a feed rate of 10 g/min. The fired product was oxidized by heat treatment at 900°C while air was circulating through the kiln at 45 L/min, yielding a powdered heat-treated product (oxidation step). This heat-treated product was designated the powder of Example 1. The rotary kiln had an inclination angle of 1° and a rotation speed of 1.0 rpm. The residence time (heating time) of the fired product in the kiln was 90 minutes. The carbon content of the resulting powder was 1.8% by mass, oxygen content 1.8% by mass, boron content 42% by mass, and nitrogen content 54% by mass. The average particle size of the resulting powder was 35 μm.

<Evaluation of powder>
The obtained powder was measured by Raman spectroscopy to determine the maximum peak value A in the wave number range of 1480 cm ⁻¹ or more and 1900 cm ⁻¹ or less, and the maximum peak value B in the wave number range of 1000 cm ⁻¹ or more and less than 1480 cm ⁻¹ , and the intensity ratio (value A/B) was determined.

Example 2
1000 g of the fired material prepared in the same manner as in Example 1 was loaded into an alumina crucible and heat-treated in an air atmosphere at 800°C for 1 hour in a muffle furnace. After 3 hours, the temperature was lowered to 150°C, and the powder was stirred so that the heat-treated surface material loaded in the crucible and the fired material inside the crucible were interchanged. Thereafter, the powder was further heat-treated in an air atmosphere at 800°C for 1 hour in a resistance heating furnace. Similarly, the temperature was lowered to 150°C and heat-treated again at 800°C for 1 hour to obtain a powdered heat-treated product (oxidation step). This heat-treated product was designated as the powder of Example 2. The carbon content, oxygen content, and average particle size of the obtained powder were measured in the same manner as in Example 1. The obtained powder was also analyzed by Raman spectroscopy in the same manner as in Example 1 to determine the A/B value. The results are shown in Table 1.

Example 3
Powder was obtained in the same manner as in Example 1, except that the temperature of the kiln furnace was changed to 700°C. The carbon content, oxygen content, and average particle size of the obtained powder were measured in the same manner as in Example 1. The obtained powder was also analyzed by Raman spectroscopy in the same manner as in Example 1, and the A/B value was determined. The results are shown in Table 1.

Example 4
Powder was obtained in the same manner as in Example 1, except that the kiln temperature was changed to 1000°C. The carbon content, oxygen content, and average particle size of the obtained powder were measured in the same manner as in Example 1. The obtained powder was also analyzed by Raman spectroscopy in the same manner as in Example 1, and the A/B value was determined. The results are shown in Table 1.

Example 5
A powder was obtained in the same manner as in Example 1, except that the boron carbide pulverization method was intensified to 150 minutes and boron carbide powder classified into fine powder with an opening size of 32 μm or less was used. The carbon content, oxygen content, and average particle size of the obtained powder were measured in the same manner as in Example 1. The obtained powder was also analyzed by Raman spectroscopy in the same manner as in Example 1, and the A/B value was determined. The results are shown in Table 1.

Example 6
A powder was obtained in the same manner as in Example 1, except that the method of preparing the boron carbide after pulverization was changed to use boron carbide powder classified using a sieve with a mesh size of 45 μm or more and 212 μm or less. The carbon content, oxygen content, and average particle size of the obtained powder were measured in the same manner as in Example 1. The obtained powder was also analyzed by Raman spectroscopy in the same manner as in Example 1, and the A/B value was determined. The results are shown in Table 1.

(Comparative Example 1)
The powdered fired product obtained in Example 1 (before the oxidation step) was used as the powder of Comparative Example 1. The carbon content, oxygen content, and average particle size of the obtained powder were measured in the same manner as in Example 1. The obtained powder was also analyzed by Raman spectroscopy in the same manner as in Example 1 to determine the A/B value. The results are shown in Table 1.

<Performance evaluation as a powder raw material>
Using the powders obtained in Examples 1 to 6 and Comparative Example 1 as raw materials, hexagonal boron nitride powder was produced and the productivity was evaluated.

Boric acid was added to the powder and mixed using a Henschel mixer. 2 kg of the mixture was then loaded into a boron nitride crucible and heated from room temperature to 2000°C in a nitrogen gas atmosphere under atmospheric pressure in a resistance heating furnace. At 2000°C, sufficient crystallization was achieved to produce hexagonal boron nitride powder. The amount of boric acid added was the minimum amount necessary to achieve a graphitization index of 2 or less for the hexagonal boron nitride powder obtained by firing at 2000°C. The time required to maintain the temperature at 2000°C (firing time) and the total time required from the start of heating until the temperature returned to room temperature (heating time) were the times required for sufficient crystallization at 2000°C. The amount of boric acid added, firing time, and heating time for Examples 1 to 6 and Comparative Example 1 are shown in Table 1.

When hexagonal boron nitride powder was prepared as described above, the yield of the hexagonal boron nitride powder was evaluated based on the raw material powder, as well as the production efficiency, taking into account the time required for production (heating time). The results are shown in Table 1. Note that the production efficiency refers to the value obtained by dividing the yield by the heating time, and is shown in Table 1 as a relative value based on Comparative Example 1.

[Graphitization index of hexagonal boron nitride powder]
The graphitization index of the hexagonal boron nitride powder used to determine the boric acid content was calculated from the results of measurement by powder X-ray diffraction. In the obtained X-ray diffraction spectrum, the integrated intensities of each diffraction peak corresponding to the (100), (101), and (102) planes of the primary particles of hexagonal boron nitride (i.e., each diffraction peak) and its baseline were calculated (arbitrary units), and these were designated S100, S101, and S102, respectively. Using these calculated areas, the graphitization index was determined based on the following formula (1):
GI=(S100+S101)/S102... Formula (1)

This disclosure provides a powder containing particles composed primarily of carbon, nitrogen, and boron and having the same crystal structure as turbostratic boron nitride, which can improve the productivity of hexagonal boron nitride, as well as a method for producing the powder.

Claims (5)

A powder containing particles having the same crystal structure as turbostratic boron nitride, the particles containing carbon, nitrogen, boron, and oxygen as constituent elements,
A powder in which, in a Raman spectroscopy spectrum, the maximum value of a peak in the wavenumber range of 1480 cm ⁻¹ or more and 1900 cm ⁻¹ or less is defined as A, and the maximum value of a peak in the wavenumber range of 1000 cm ⁻¹ or more and less than 1480 cm ⁻¹ is defined as B, and the value of A/B is 1.4 or less. The powder described in claim 1, having a carbon content of 4.0% by mass or less and an oxygen content of 5.0% by mass or less. The powder according to claim 1 or 2, having an average particle size of 10 to 100 μm. A method for producing a powder containing particles having the same crystal structure as turbostratic boron nitride, the particles containing carbon, nitrogen, boron, and oxygen as constituent elements, comprising:
a pressure nitriding step of sintering the boron carbide powder at a temperature of 1900 to 2200°C in a pressurized nitrogen atmosphere to obtain a sintered product;
an oxidation step of heat-treating the fired product at a temperature of 700 to 1050°C in an atmosphere having an oxygen partial pressure of 20% or more to obtain a heat-treated product;
The oxidation step comprises stirring a mixture containing the fired product and the heat-treated product. The method according to claim 4 , wherein the mixture is stirred by a rotary kiln.